Totally reflecting laser refractometer

ABSTRACT

A laser having isotropic polarization characteristics, is operated with a confocal mirror cavity and is adjusted in terms of mirror reflectivity, spacing, and laser active medium to operate at a particular wavelength. A total internal reflection prism is contained inside the laser optical cavity and is rotated to produce an optical path angle slightly greater than the critical angle. For this condition the prism will constrain the laser to oscillate in such a way that adjacent modes have orthogonal polarizations. In the case of two mode operation one mode will be polarized parallel to the plane of incidence at the prism reflecting surface and the other will be perpendicular to the first. The phase shifts produced upon total reflection are different for light polarized parallel and perpendicular to the plane of incidence. Futhermore the difference in the phase shifts for these two polarization directions is a function of the refractive indices of the prism and its facing material as well as the angle of incidence. Since the difference in phase shift is equivalent to a difference in optical cavity length for the two orthogonally polarized modes the frequency of one of the modes is slightly displaced with respect to the other. Measurement of the beat frequency difference between the orthogonal modes will therefore yield information on the refractive index of the facing material provided that prism index and angle of incidence are known. In one embodiment the prism is provided with a calibrating sample of known refractive index placed in close contact with its reflecting face. The prism and laser cavity are then adjusted to produce a stable mode difference beat frequency. Very small changes in refractive index of the material in contact with the prism face can be correlated with changes in beat frequency thereby giving a very sensitive measure of refractive index. If the comparison sample is a fluid confined inside a chamber sealed to the prism face, extremely slight differences in composition of the fluid can be detected. Since the output is read immediately as a frequency, the system is useful in monitoring flowing fluids.

United States Patent [1 1 White Nov.6, 1973 TOTALLY REFLECTING LASERREFRACTOMETER [75] Inventor: Matthew B. White, Newport Beach,

Calif.

[73] Assignee: Philco-Ford Corporation,

Philadelphia, Pa.

22 Filed: May 8,1972 [21] Appl. No.: 250,877

52 us. Cl. 356/133, 356/136 [51]- Int. Cl. G0ln 21/46 [58] Field ofSearch 356/134, 135, 136, 356/118; 331/945 A [56] References CitedUNITED STATES PATENTS 3,506,362 4/1970 Doyle et al. 331/945 A 3,060,79310/1962 Wells 356/118 3,526,771 9/1970 l-ienkel et al. 356/135 PrimaryExaminerDavid Schonberg Assistant Examiner-Conrad Clark Attorney-RobertD. Sanborn et al.

in such a way that adjacent modes have orthogonal polarizations. In thecase of two mode operation one mode will be polarized parallel to theplane of incidence at the prism reflecting surface and the other will beperpendicular to the first. The phase shifts produced upon totalreflection are different for light polarized parallel and perpendicularto the plane of incidence. Futhermore the difference in the phase shiftsfor these two polarization directions is a function of the refractiveindices of the prism and its facing material as well as the angle ofincidence. Since the difference in phase shift is equivalent to adifference in optical cavity length for the two orthogonally polarizedmodes the frequency of one of the modes is slightly displaced withrespect to the other. Measurement of the beat frequency differencebetween theorthogonal modes will therefore yield information on therefractive index of the facing material provided that prism index andangle of incidence are known.

In one embodiment the prism' is provided with a calibrating sample ofknown refractive index placed in close contact with its reflecting face.The prism and laser cavity are then adjusted to produce a stable modedifference beat frequency. Very small changes in refractive index of thematerial in contact with the prism face can be correlated with changesin beat frequency thereby giving a very sensitive measure of refractiveindex. If the comparison sample is a fluid confined inside a chambersealed to the prism face, extremely slight differences in composition ofthe fluid can be detected. Since the output is read immediately as afrequency, the system is useful in monitoring flowing fluids.

10 Claims, 5 Drawing Figures AAMZYZIR 4/v0/ on ITO/7,4 6:

Ana 0U j TOTALLY REFLECTING LASER REFRACTOMETER BACKGROUND OF THEINVENTION Refractometry hasbecome an extremely useful tool in the artsof chemical analysis and process control; It has been found thatsolution concentration for specified materials can be preciselydetermined by measuring the refractive index of thecombination. Dilutionto a specified refractive index can be employed as a process control foreither batch or continuous process operations. I

Numerous refractometer designs have been developed to implementrefractive index measurements for various applications. For example, thematerial tobe measured is placed in close contact with a standard ma--terial and the critical angle for total reflection is measured.Alternatively, image displacement can be mea sured where the light iscaused to pass the interface between standard and unknown materials at asuitable angle'. Retardationin an unknown sample can be measured byinterference where a monochromatic beam of light is passed through thesample and then beat against a reference beam to produce interferencefringes.

In another system'a series of beads made of materials of known gradedrefractive indices is immersed in a liqui'd to be measured..lf anyparticular bead matches therefractive index of the liquid it will seemto disappear. The least discernable bead is the one most nearly matchingthe index of the liquid. This technique can be enhanced by making thebeads into a series of rods that act as light guides between a lightsource and photo detector. The guide showing the lowest transmission isthe one most nearly matching the liquid index.

In all the above methods, except for the critical angle technique, lightmust be passed through both the reference and sample materials. Thushigh sample transparency is ordinarily required and large samples areoften required to achieve reasonable accuracy. Using the critical anglemeasurement, poorly transmitting samples can still be measured with goodaccuracy. This is due to thefact that slightly above the critical anglefor reflection, the light only passes a short distance into the samplematerial and only the reference material need be highly transparent.

In refractometry, the important measurement factors include small samplesize, accuracy, ease and speed of readout, and sensitivity. In order toachieve desired performance several of the above techniques have oftenbeen combined in prior art machines because no single techniqueordinarily combines all of the best features.

SUMMARY OF THE INVENTION It is an object of the invention to measurerefractive index on a small sample of material to a high degree ofaccuracy and sensitivity very rapidly.

' lar optical polarization. The laser medium is selected to It is afurther object to facilitate such measurements I on samples having pooroptical transparency.

It is a still further object of the invention to use a laser source inthe measurement of refractive index where I adjacent mode beat frequencyis used as the output inhave a transition that produces optical energyin the wavelength desired for the measurement. 'The laser cavity isestablished to support oscillation at the selected wavelength andusually employs confocal mirrors, one of which is slightly transparent.Included in side the optical cavity is a reflecting prism, of knownrefractive index, that bends the cavity path so that reflection from theprism face at near the critical angle isnecessary to couple the cavitymirrors. The sample to be evaluated is placed in intimate contact withthe reflective prism face. Near the critical angle the prismsamplecombination introduces anisotropy in the form of an effectivebirefringence into the laser system so as to produce dual-polarizationoperation. Laser oscillation can occur. in only two polarization statesbecause the constructive interference required to produce stable lasermodes can onlyoccur for the polarization parallel to the plane ofincidence at the prism reflecting surface and for the orthogonalpolarization. All other polarization conditions result innonconstructive interference thereby preventing laser oscillation forthese conditions. Where laser oscillation is constrained to two adjacentoscillating modes they will have exactly orthogonal polarizations thatlie in directions that are parallel and perpendicular to the prism planeof incidence. Since the total internal reflection process produces adifferential phase shift for light polarized in these mutuallyorthogonal directions an effective differential optical cavity length(i.e. birefringence) is introduced for the two modes. The modes willtherefore operate at different frequencies with the differencedetermined by the magnitude of the differential phase shift. The amountof differential phase shift is a known function of theangle-of-incidence, refractive index of the prism, and refractive indexof the sample. Hence, if the angle-of-incidence and prism index areknown a measure of the mode beat frequency difference can be correlatedwith the sample refractive index. Since frequency measurements can bemade quite accurately a I very sensitive refractive index measure isavailable.

BRIEF DESCRIPTION OF TI-IE DRAWING DESCRIPTION OF THE EQUIPMENT In FIG.1 of the drawing a laser 1 having suitable pumping or excitation means 2is located inside an optical cavity the axis of which is indicated bythe bent transitions for Operating in the vicinity of 0.633, 1.15, and3.39 microns. I-Ie-Xe operates well at 2.03 and 3.5 microns. CO isuseful at 10.6 microns. In general, laser materials are available fromnear ultra violet to far infra red. Some, like the organic dye liquids,are easily tunable over a broad range of frequencies. The laser shoulddesirably be capable of operating continuously at low power but, withsuitable measurement circuitry, could be pulsed.

The preferred laser 1 is made isotropic in that it will accommodatelight of any polarization. The laser end windows are made slightly outof parallel to prevent their acting as resonators, but the departurefrom parallelism is not sufficient to introduce polarizing effects.Desirably the interior of the laser tube is made a diffuse reflector todiscourage wall-reflected oscillatory modes. Such a laser is describedmore completely in my U.S. Pat. No. 3,500,233.

Prism 4 is mounted on rotary platform 5 so that angle 0 can be adjustedto the critical value. Mirror 6 is made to have high reflectivity overthe wavelengths to. be used and is mounted on rotary platform 7 so thatits angle relative to the axial path 3 can be adjusted to accommodatevarious angles of prism 4. Mirror 8 is similar to mirror 6 but is madepartly transmissive (on the order of l to 2 percent) to allow efficientlaser operation yet couple out a small portion of the laser energy.

It can be seen that the two mirrors provide resonator action, and thelaser with its pumping source provides the negative temperature mediumfor oscillation. Since the optical path includes prism 4, anisotropy isintroduced into the otherwise isotropic system. Theprism which totallyreflects optical energy that strikes its reflective back face will bethe dominant polarizationcontrolling element in the system. Only opticalenergy that is polarized either parallel or perpendicular to theplane-of-incidence at the prism surface can achieve a stable stationarystanding wave pattern or mode in the optical resonator. Hence all otherpolarization states are excluded.

Photodetector 9 senses the laser output from mirror 8 through apolarizer; Polarizer 10 is oriented at about 45 degrees with respect tothe laser polarization states and mixes the orthogonal output signals sothat photodetector 9 will heterodyne them. The output from thephotodetector will contain a dc component representing the laser lightintensity and an a-c component representing the beat frequencydifference between the laser modes. Analyzer 11 may simply be afrequency indicator or spectrum analyzer, and it may further includeelements that convert the frequency indication into refractive indexindications. In the latter case prism angle 6 information in the form ofdashed line 12 must be fed into the analyzer.

Sample 13, the refractive index of which is to be evaluated can be inthe form of a solid, liquid, or gas. In

solid form the sample face should be polished flat so as to intimatelyconform to the mating prism face as shown in FIG. 2. Screw clamp 14holds the sample in place on the prism face. Since the opticalpenetration into the sample is small for total internal reflection, thesample can be made quite thin. The sample lateral dimensions need onlybe large enough to cover the laser active spot on the prism face. Thisis ordinarily on the order of a few millimeters in diameter. If thesample is more nearly equal to the size of the beam it must beaccurately positioned with respect to the beam. Larger samples do notrequire accurate positioning.

Where the sample is a liquid or gas, a confining cell is mounted on theprism face so that the sample is in direct contact therewith as shown inFIG. 3. The sample cell can be filled and sealed off. If desired,variable pressure means can be included in its structure. Inlet andoutlet flow channels can alsobe provided for rapid sample changing.

As shown in FIG. 4 the prism 4 can be sealed into a wall of a processconduit 14 carrying a flowing fluid that is to be evaluated. In thiscase the fluid flowing in the conduit should be homogeneous and the flowrate such that turbulence in the vicinity of the prism avoided.

FIG. 5 shows how the laser refractometer can be used in process control.The refractometer of FIG. 1 is associated with a fluid transfer line 14-such as the one shown in FIG. 4 and is designated as 15. An inputprocessing station 16 could for example mix two or more liquids fortransfer to an output processing station 17 which could be for example abottle filling device. The mixing process is to be controlled foruniformity of produce bottled. The desired mixture in line 14 has aparticular refractive index and refractometer 15 is adjusted to providea known output for this value. If the refractive index in the fluid fromstation 16 changes, refractometer 16 will feed a different signal toservo 18 which then adjusts the process to restore the originalrefractive index. Since the refractometer will respond strongly to verysmall changes in refractive index, excellent control over the processcan be maintained.

Prism 4 should be homogeneous, transparent, and of high optical quality.The sample and optical faces should be polished flat. The prism materialwill depend upon the operating laser wavelength. For visible light glassor quartz may be used. For near infra red, quartz or sapphire may berequired. Far infra red may call for such materials as germanium,silicon, rock salt, or one of the Irtran series of materials. In generalit is necessary that the prism be transparent to the operating lightwavelength, mechanically capable of coping with the conditions of use,andchemically inert to any sample materials that may contact it.

OPERATION OF THE INVENTION The output of a dual polarization lasercomprises two orthogonally polarized components having frequencies :1and v The frequency difference or beat frequency is u, u Av and is givenby the expression:

where:

C speed of light in meters per second L length of laser cavity in meters:1), total phase shift of component 1 for a single pass through thecavity 111 total phase shift of component 2 for a single pass throughthe cavity Av, the a-c component of the photo detector 9 (FIG. 1)output, can be easily measured to high precision.

Simple total internal reflection theory indicates that, neglectingoptical absorption, the system of FIG. 1 will have a differential phaseshift 4: d), between the orthogonally polarized modes given by:

Equations 3 and 4 show that the sensitivity of the system is maximumnear the critical angle where sin N,/N,,. By substituting sin 0 z N,/Nand Av ('n'L/C z 0, equation (4) reduces to:

If a 1 meter laser cavity is used with asample having an index of 2against a germanium prism (N 4), and assuming that 0 has been adjustedto produce a Av of 1 MHz, equation 5 reduces to:-

Equation (6) shows that a fluctuation of 0.00001 in sample refractiveindex would give rise to a 150 KHz variation in detector outputfrequency. Such a measurement could be made in a small fraction of amillisecond. This shows that the system is capable of monitoring verysmall changes in refractive index in a continuous manner for a flowingsystem. Thus it is readily adaptable to servo-controlled processsystems.

In a carbon dioxide dual polarization laser system operating at about10.6 microns, measurement of the refractive index difference betweenhelium and air was made. The prism material was NaCl and was first facedwith an air sample at 10 p.s.i. The prism was rotated for a beatfrequency output of 1.7 MHz. A shift of 0.45

terms of prism index, 0, cavity length, and beat frequency. Howeversince the ideal conditions, including complete sample transparency, areseldom met in practice where less than ideal conditions occur,corrections must be applied to the formula to obtain a high degree ofaccuracy. Where a series of measurements are to be taken, it is morepractical to calibrate the device in terms of samples of knownrefractive index. First the system is adjusted so that dual polarizationoperation occurs over the intended sample range and the rotary elementlocked in place. Then vthe beat frequency is noted for known indexsamples in the range of interest. A calibration curve is then plottedfor the known samples using at least three points for any'givencalibration. Unknown samples can then be located on the curve by simplyreading the beat frequency and reading out refraetive index from thecurve.

While a laser system and certain alternative active device forms andseveral sample arrangements have been described, numerous alternativeswill occur to a person skilled in the art. For example the photodetector and polarizer could be combined with either laser mirror whichwould be made slightly transmissive. Both mirrors could be made slightlytransmissive or both could be made highly reflective with some othermeans, such as a beam splitting mirror, employed to extract a signal forthe photo detector. Furthermore one of the laser mirrors couldbecombined with the laser active device structure. Mirror configurationsother than confocal could be employed. The invention is intended to belimited only by the following claims.

I claim:

l. A refractometer comprising:

a. a laser,

b. means associated with said laser to produce dualpolarizationoperation, said means comprising a prism rotated to produce totalinternal reflection of the light signal in said laser,

c. means for converting a portion of the output of said laser to anelectrical signal having a frequency equal to the difference infrequency between dual polarization laser modes, and

d. means for maintaining in close optical contact with the reflectingface of said prism, a sample of material to be evaluated, whereby saidelectrical signal is a function of the ratio of the refractive index ofsaid sample to the refractive index of said prism.

2. The refractometer of claim 1 wherein said means for maintaining isadapted to handle a solid sample.

3. The refractometer of claim 1 wherein said means for maintaining isadapted to handle a fluid sample.

4. The refractometer of claim 3 wherein said means for maintaining isadapted to handle a fluid sample that is in motion.

5. The refractometer of claim 4 wherein said sample is being processedand said electrical signal is used in MHz was observed when helium at 10p.s.i. was substituted for the air in the sample cell. This indicated achange in index of 0.000065, a value that is consistent with a valueobtained by extrapolating published data for the visible portion of thespectrum.

As shown particularly by equation (3), absolute measurements of samplerefractive index can be made in a process control system.

6. A refractometer comprising: a. an optical resonant cavity having awell defined optical axis, b. a laser active medium contoured to avoidfavoring any polarization state of optical energy therein, and locatedinside said resonant cavity, c. means for pumping said laser activemedium to a level sufficient to establish laser action along saidoptical axis,

d. means for producing dual-polarization laser action by introducingpolarization anisotropy inside said cavity comprising:

1. an optical prism mounted in said cavity and oriented so that saidoptical axis approaches a face inside said prism near the critical anglefor total internal reflection and 2. means for locating a sample to betested in intimate contact with said face of said prism,

e. means for extracting a portion of the optical energy in said cavity,and

f. means for converting the optical signals resulting from saidanisotropy into an electrical signal having a frequency related to therefractive index of said sample.

7. The refractometer of claim 6 wherein said means for locating isadapted to accommodate a solid sample having at least one flat polishedface thereon to com form with said face of said prism.

8. The refractometer of claim 6 wherein said means for locating isadapted to maintain a sample fluid in contact with said face of saidprism by means of an enclosure mounted on said prism.

9. The refractometer of claim 8 wherein said means for locating isfurther adapted to contain a fluid in motion in a fluid process and saidmeans for converting of clause (f) provides a continuous readout relatedto the refractive index of said fluid.

10. The refractometer of claim 9 wherein said fluid emanates from aprocess control operation regulated by a servomechanism, saidrefractometer providing control information to said servomechanism tostabilize the refractive index of said fluid in said process.

a: y a:

1. A refractometer comprising: a. a laser, b. means associated with saidlaser to produce dual-polarization operation, said means comprising aprism rotated to produce total internal reflection of the light signalin said laser, c. means for converting a portion of the output of saidlaser to an electrical signal having a frequency equal to the differencein frequency between dual polarization laser modes, and d. means formaintaining in close optical contact with the reflecting face of saidprism, a sample of material to be evaluated, whereby said electricalsignal is a function of the ratio of the refractive index of said sampleto the refractive index of said prism.
 2. The refractometer of claim 1wherein said means for maintaining is adapted to handle a solid sample.2. means for locating a sample to be tested in intimate contact withsaid face of said prism, e. means for extracting a portion of theoptical energy in said cavity, and f. means for converting the opticalsignals resulting from said anisotropy into an electrical signal havinga frequency related to the refractive index of said sample.
 3. Therefractometer of claim 1 wherein said means for maintaining is adaptedto handle a fluid sample.
 4. The refractometer of claim 3 wherein saidmeans for maintaining is adapted to handle a fluid sample that is inmotion.
 5. The refractometer of claim 4 wherein said sample is beingprocessed and said electrical signal is used in a process controlsystem.
 6. A refractometer comprising: a. an optical resonant cavityhaving a well defined optical axis, b. a laser active medium contouredto avoid favoring any polarization state of optical energy therein, andlocated inside said resonant cavity, c. means for pumping said laseractive medium to a level sufficient to establish laser action along saidoptical axis, d. means for producing dual-polarization laser action byintroducing polarization anisotropy inside said cavity comprising: 7.The refractometer of claim 6 wherein said means for locating is adaptedto accommodate a solid sample having at least one flat polished facethereon to conform with said face of said prism.
 8. The refractometer ofclaim 6 wherein said means for locating is adapted to maintain a samplefluid in contact with said face of said prism by means of an enclosuremounted on said prism.
 9. The refractometer of claim 8 wherein saidmeans for locating is further adapted to contain a fluid in motion in afluid process and said means for converting of clause (f) provides acontinuous readout related to the refractive index of said fluid. 10.The refractometer of claim 9 wherein said fluid emanates from a processcontrol operation regulated by a servomechanism, said refractometerproviding control information to said servomechanism to stabilize therefractive index of said fluid in said process.